In a charged particle beam apparatus, such as an electron beam lithography unit, a scanning electron microscope, and a transmission electron microscope, the electron beam hits a shaping aperture, the image of which is demagnified and projected onto a target, such as a mask or a wafer. Although the surface of the aperture can be baked out, so that a contaminant, such as an oxide, on the surface of the aperture is cleared prior to use, hydrocarbon gas molecules present in residual gas molecules or from grease at sealed joints are polymerized and deposited on the surface of the aperture which is hit by the electron beam. Such contamination can accumulate to the point that the critical dimensions of the aperture change causing the aperture to be imaged improperly on the target. Charge which builds up on a layer of contamination causes beam deflection resulting in a positioning error.
Many efforts have been made to protect against the accumulation of contaminants on the surface of the aperture. None of the many efforts appear, however, to have solved the problem to the extent desired. Therefore, a need remains for an improved charged particle beam apparatus having structure preventing contamination.